A law enforcement official says no active shooter was found at a military post outside Washington.

The law enforcement official was not authorized to discuss the matter by name and spoke on condition of anonymity.

Joint Base Andrews had been placed on lockdown Thursday morning after an active shooter was reported. The base is home to Air Force One and is about 20 miles from Washington.

An active shooter exercise had also been planned at the base, but officials said the situation became "real world."

The situation unfolded at a medical facility on the post. Joint Base Andrews issued a tweet telling all personnel at the base in Washington's Maryland suburbs to shelter in place.

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10:20 a.m.

Emergency vehicles with lights on but no sirens are in the area of a military base on lockdown after an active shooter was reported on the post.

At least three people in camouflage and helmets could be seen walking working dogs around the three-story medical building at Joint Base Andrews. The post is home to Air Force One and is about 20 miles outside Washington.

The base was put on lockdown Thursday morning after an active shooter was reported. An active shooter exercise had been planned, but officials say reports of a real-world active shooter came in.

Around 10:15 a.m., a few people could be seen walking out of the building, including a person being wheeled in a wheelchair.

Helicopters hovered overhead, but it is unclear if they are news or military helicopters.

10:10 a.m.

Officials say first responders are trying to secure the scene of an active shooter report at a military post near Washington.

Joint Base Andrews, which is about 20 miles outside the nation's capital and is home to Air Force One, said in a tweet Thursday that the base is reacting to the active shooter report to ensure the safety of all personnel.

It was not immediately clear if any shots were fired or if anyone was wounded.

Meanwhile, Homeland Security Secretary Jeh Johnson says there is an "unfolding situation" at Joint Base Andrews in Maryland but is providing no further details.

The base tweeted Thursday morning that it was on lockdown after reports of an active shooter.

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10 a.m.

Homeland Security Secretary Jeh Johnson says there is an "unfolding situation" at Joint Base Andrews in Maryland but is providing no further details.

The base, which is home to Air Force One, tweeted Thursday that the base is on lockdown due to a report of an active shooter.

The tweet instructs all personnel at the base in Washington's Maryland suburbs to shelter in place and says more information will be released as it comes.

Johnson is testifying before the Senate Judiciary Committee. He says he may have to take a break from the hearing as the situation at Andrews develops.

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9:55 a.m.

An employee at a medical facility at Joint Base Andrews outside Washington says an active-shooter drill had been planned, but officials later told workers it was a "real-world" situation.

Rodney Smith is a patient advocate at the military post's medical facility. He said Thursday that he was on lockdown and didn't have any more information.

Smith said by phone: "First it was an active-shooter exercise. Then it came back 'real world.'"

Smith said the situation was unfolding at the newer of two buildings at the Malcolm Grow Medical Facility on the base. He was in the older building.

Joint Base Andrews said in a tweet that the incident is ongoing and that first responders are on scene.

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9:50 a.m.

Joint Base Andrews tweets that the base is on lockdown due to a report of an active shooter.

The tweet sent Thursday morning instructs all personnel at the base in Washington's Maryland suburbs to shelter in place and says more information will be released as it comes.

A second tweet from the base says the incident is ongoing at the Malcolm Grow Medical Facility and first responders are on the scene. It instructs everyone to continue to shelter in place, a precaution meant to keep people safe while remaining indoors.

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